So there turned out to be an additional requirement that I realised later I hadn't spelled out : availability of external spare battery charger in India. With this my list narrowed down to the Zenfone Selfie and the Galaxy S4. Since the S4 is quite old, I went ahead and ordered the Zenfone Selfie; I received it last night. It seems as good as expected. The spare battery+charger (Battery Charger (ACHA001) | Phone Accessory | ASUS India) has been ordered, should reach me soon.

So that's the end of a month-long search. Let's hope it turns out to be as tweaker-friendly as it seems to be.